SUMMARY Relationships between crystal chemistry and genetic conditions are presented for a series of dioctahedral micaceous minerals. Using a ferruginicity index, three groups of micaceous minerals are distinguished: Al‐micas, Fe‐illites, and glauconites. The degree of Al‐for‐Si substitution in the tetrahedral sheets of micaceous minerals depends on their thermodynamic genetic conditions. If pressure and temperature increase, the Al‐IV content increases and the octahedral sheet is “cleaned” from divalent cations. This process is accompanied by structural alteration with a polytype sequence 1Md‐1M‐2M1. Tetrahedral and octahedral charges are the same for micaceous minerals formed at the surface (sedimentogenesis/diagenesis) and are independent of the iron content. For most illites of sedimentary rocks an authigenic origin is suggested.

SUMMARY This paper presents the main trends in epigenetic research in the U.S.S.R. in the last years: Study of epigenetic zonality in thick series of sedimentary rocks of various ages and tectonic regions, (zones of unaltered clay matrix, of altered clay matrix, of quartzitic structures and hydromica‐chloritic cement, and of prick‐like structures and phyllite‐like schists). Principles of stadial analysis and mineral indicators of different epigenetic stages (clay minerals, organic matter, zeolites, feldspars, sulfides). Facies of regional epigenesis connecting the main petrographic families of unaltered terrigenic rocks (quartz sandstones with kaolinitic matrix and the same composition of associated clays, acidic arkoses with hydromicaceous clays, lithoclastic sands with several matrix components and associated clays, and volcanogenic greywackes) with “related” facies of regional metamorphism. Some practical aspects of epigenesis are: for petroleum geology: maximum depth of burial according to porosity, interruption of quartz, feldspar, sphene, epidote, carbonate, biotite, and clay mineral epigenesis by oil migration; and for ore geology: (a) initial epigenesis: ore impregnation (e.g., volkonskoite in wood remains) due to intrastratal solution of heavy minerals and leaching of rock fragments; (b) deep epigenesis: e.g., galena impregnations due to release of trace elements during dissolution of rock forming minerals e.g., feldspars; and (c) metagenesis: “Alpine” veins across and along bedding, filled, e.g., with copper ores.

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